Instruments for determining true horizontal and vertical range from the carpenter's level and plumb bob which have been used nearly since antiquity through precise opto-mechanical instruments, such as optical transit squares, to electro-mechanical instruments such as gyroscopes. Each type of instrument is adequate for a given application with regard to required precision, speed of measurement, and convenience. The majority of the simpler inclination indicating instruments do not provide a quantitative indication of inclination, but merely an indication of whether a test surface is level. Such an indication is adequate for constructing relatively small structures, such as residential buildings of several stories or less, but much greater accuracy is required for building relatively large structures such as skyscrapers, oceangoing vessels and the like.
When a high degree of accuracy is required, optical tooling instruments are often used, such as optical transit squares, by means of which critical points, planes, and surfaces are observed through precise telescopes and related to reference lines or planes. While much greater accuracy can be achieved with such instruments, the actual use of such instruments can be very time consuming. Another disadvantage of such instruments is that their usefulness is decreased when used in moving frames of reference such as on a ship or floating platform at sea.
The use of electrical sensors in cooperation with Wheatstone bridges, Maxwell bridges, and other types of bridge circuits in inclination measuring instruments provides more convenience in determination of inclination without significantly sacrificing accuracy. Most such bridge circuits have been employed with analog type meters which have generally non-linear response across their scales and have a maximum accuracy of about three percent. For this reason, when used with analog type meters, nulling, zeroing, and bridge balancing techniques are generally employed for greatest accuracy. This requires adjusting and reading calibrated dials or switching decade type resistors for the nulling procedure. Such adjustments or switching is also time consuming and in some cases prone to operator error.
Further, gyroscopic inclination sensors have been employed particularly in moving frames of reference such as aircraft and ships. Such instruments are quite adequate for their intended purpose, mainly the indication of relative inclination. However, gyroscopic inclination sensors are subject to error as a result of bearing friction such that they are not adequate for precise measuring.
In some circumstances, it is desirable to monitor the inclination of many portions of a structure. For example, large ships and marine platforms are often subject to flexure of spaced sections due to wave action and thermal cycles as well as movement of the structures as a whole. In such cases, it is desirable to monitor the degree of flexure for safety. There is an ongoing requirement for the alignment of substructures such as propulsion and navigation equipment, certain tools and instruments, and on naval vessels, weapons structures. One requirement for meaningful data in multiple sensor systems is that the taking of readings must be synchronized. Otherwise, it is impossible to accurately compare readings. Such synchronization is most practically achieved in digital inclination measuring systems.